A protein kinase inhibitor is an enzyme inhibitor that blocks the action of a protein kinase. A protein kinase is an enzyme that adds a phosphate group to a protein or other organic molecule. Phosphorylation is involved in a wide range of diseases, such as diseases associated with aberrant activity (e.g., increased activity) of a protein kinase. Such diseases include, but are not limited to, proliferative diseases (e.g., cancers, benign neoplasms, pathological angiogenesis, inflammatory diseases, and autoimmune diseases). Inhibiting protein kinases, and therefore the phosphorylation of a substrate protein, has been shown to be useful in treating these diseases. For example, afatinib, an ErbB inhibitor, is useful in treating non-small cell lung cancer; axitinib, a VEGFR, PDGFR, and c-KIT inhibitor, is useful in treating renal cell carcinoma; bosutinib, a Bcr-Abl inhibitor, is useful in treating chronic myelogenous leukemia; cabozantinib, a c-Met and VEGFR2 inhibitor, is useful in treating thyroid cancer; crizotinib, an ALK, HGFR, and c-MET inhibitor, is useful in treating non-small cell lung cancer; dasatinib, a Bcr-Abl, Src, and c-KIT inhibitor, is useful in treating chronic myelogenous leukemia; erlotinib, an EGFR inhibitor, is useful in treating non-small cell lung cancer and pancreatic cancer; gefitinib, an EGFR inhibitor, is useful in treating non-small cell lung cancer; imatinib, a Bcr-Abl inhibitor, is useful in treating chronic myelogenous leukemia; lapatinib, a HER2 inhibitor, is useful in treating breast cancer; nilotinib, a Bcr-Abl inhibitor, is useful in treating chronic myelogenous leukemia; pazopanib, a VEGFR, PDGFR, and c-KIT inhibitor, is useful in treating renal cell carcinoma and soft tissue sarcoma; ponatinib, a Bcr-Abl, BEGFR, PDGFR, FGFR, EPH, SRC, c-KIT, RET, TIE2, and FLT3 inhibitor, is useful in treating chronic myelogenous leukemia and acute lymphoblastic leukemia; regorafenib, a RET, VEGFR, and PDGFR inhibitor, is useful in treating colorectal cancer and gastrointestinal stromal tumor; ruxolitinib, a JAK inhibitor, is useful in treating myelofibrosis; sorafenib, a VEGFR, PDGFR, BRAF, and c-KIT inhibitor, is useful in treating renal cell carcinoma and hepatocellular carcinoma; sunitinib, a VEGFR and PDGFR inhibitor, is useful in treating renal cell carcinoma, gastrointestinal stromal tumor, and pancreatic neuroendocrine tumor; tofacitinib, a JAK inhibitor, is useful in treating rheumatoid arthritis; vandetanib, a VEGFR, EGFR, RET and BRK inhibitor, is useful in treating thyroid cancer; and vemurafenib, a BRAF inhibitor, is useful in treating malignant melanoma. There remains a need for protein kinase inhibitors for improved treatment of diseases associated with aberrant activity of protein kinases.